1. FIELD OF THE INVENTION
This invention relates generally to electrical components, which, when exposed to extra-ordinary circumstances, will fail in a safe and controlled manner. This invention relates most specifically to electrical resistors, although one of ordinary skill may apply this invention to a variety of components and systems.
2. DESCRIPTION OF THE RELATED ART
Known electrical resistors are formed from various compositions that are electrically conductive to controlled or generally predictably varying degrees. These compositions may be deposited upon a substrate via vapor deposition, screen printing, immersion coating, or a host of other techniques, or may form the physical and the electrical structure.
Once a resistor has been formed into a suitable structure, the resistor is incorporated into a circuit using a variety of known techniques. After circuit assembly a variety of electrical and thermal energies are applied to the resistor which influence the stability of the resistor. In some instances the resistor is subjected to extreme current flow for longer time durations than the resistor can withstand. This generally will result in some form of irreversible destructive failure. The failure mode may take the form of resistance values that vary widely from the original design values, destructive separation of parts or fragments of the resistor material, or even combustion of the compositions that comprise the resistor.
In view of the observed failure modes, Original Equipment Manufacturers have established special requirements for these components. The devices must not become fire, safety or electrical hazards when subjected to extreme voltage inputs.
To meet the special OEM requirements, manufacturers of resistors have attempted to produce fail-safe resistors that incorporate one of several features known to protect the resistor and surrounding equipment. For example, prior art fusible or failsafe resistors produce an electrical and/or mechanical disconnection of power when the resistor is electrically overloaded. Some techniques for disconnection of poWer include: the physical addition of a separate and distinct fuse adjacent to the resistor; the combination of a fuse link or fuse element adjacent with or integral to the resistor; the use of a thermally sensitive substrate or device, which, when thermally stressed, produces some disconnection of power to the resistor; the use of a controlled resistance film thickness, which, when overloaded, evaporates; and other known techniques.
More recently, Taylor in U.S. Pat. No. 4,961,065, assigned to the assignee of the present invention, discloses a technique for controlling the breakage of a resistor substrate. That patent describes the manufacture of a fusible substrate by scribing the substrate at predetermined stress locations. The scribe marks cause programmed shattering of excessively thermally stressed substrates. When subjected to an overload, the substrate will fragment and interrupt the circuit continuity.
Yet, the fail-safe resistors described in the Taylor patent as well as prior art resistors that incorporated no protective function all suffer from a common drawback. When subjected to an overload of much greater energy than the fail-safe or anticipated energy, unpredictable events may occur. One particularly troublesome event is the "explosion" of the resistor substrate, be this substrate the resistive composition per se or a separate physical support. Fragmentation and associated ejection of solid material into the surrounding environment typically accompanies an "explosion." Because the safety of those workers and equipment operators who might be exposed to flying fragments is an OEM concern, OEM purchasers have determined that the prior art is inadequate to meet safety requirements.